In recent years, there have been demands for springs of high strength. In general, when seeking to increase the strength of a spring, there is a tendency for the toughness and corrosion fatigue strength, and the like, to deteriorate. Japanese Patent Application Publication No. 2008-261055 discloses a spring having a prior austenite average grain size D of not more than 20 μm, and an average martensite lath length of not more than 30% of the prior austenite average grain size D. Japanese Patent Application Publication No. 2008-261055 states that a spring having characteristics of this kind has high strength and also has high toughness. In the description given below, Japanese Patent Application Publication No. 2008-261055 is called Patent Document 1.